Tylosema esculentum is a long-lived perennial species endemic
to arid areas of
southern Africa. Its potential as a crop species
has long been recognized as a result of the high oil and protein
content of its
seeds. The reproductive biology and breeding
systems of the species were investigated in wild and experimental
populations growing in
Botswana. Field observations confirmed
that the species is heterostylous with the pistil and anthers
exhibiting reciprocal heights in the two morphs, although pollen
size and sculpturing do not vary. The wet, non-papillate stigma
characteristic of the species is the first to be reported in
the Caesalpinioideae.
In vivo and
in vitro diallel crossing
experiments demonstrated that a diallelic self-incompatability
system exists in
T. esculentum. The major site of
pollen tube inhibition in the intramorph crosses was found to be in the
style. This is the first report of functional
heterostyly in
the
Fabaceae and of a confirmed self-incompatibility system
in the Caesalpinioideae. Three separate lines of evidence, the
monitoring of fruit development in
open-pollinated plants, fruit
set in diallel crossing experiments, and observations made in
wild populations, demonstrated that fruit set and, by implication,
seed set, are very low in this species. Floral abscission was
a major limitation to the production of mature pods but there
were also significant losses at other developmental stages of
fruit production. The results suggest that low seed set may
be an adaptation of the species to an environment in which rainfall
is scarce.
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